A method for measuring mark formation effectiveness by sampling a reflected write signal at two instances in time T.sub.1 and T.sub.2 relative to an external reference timing event is described in U.S. Pat. No. 5,216,660 to Iimura, and is illustrated in FIG. 1. Such a reference timing event can be, for example, the rising edge of a write pulse sent to a recording laser. As shown in FIG. 1, the method disclosed in U.S. Pat. No. 5,216,660 uses fixed, predetermined delays T.sub.1 and T.sub.2 between the reference timing event and the times that the reflected write signal is sampled. However, there are many sources of uncertainty and variation in the actual time delay between such a reference timing event and the reflected write signal. For example, the time delay can vary from one device to another and with time and temperature for a given device. Nonadjustable sampling delays between the reference timing event and the reflected write signal may be adequate for very low data recording rates such as "1X" speed in a CD recordable ("CD-R") system, where a given sample timing error may not cause significant error in the resulting measurement. However, as data recording rates increase, that same error in sample timing can lead to larger errors when attempting to measure a specific feature of the reflected write signal, such as its peak value. This is illustrated in FIG. 2, which is a graph of reflected write signals at 1X and 8X CD-R data rates, where X is the data rate associated with CD audio playback (i.e., 4.321 Mbits per second data rate recorded to or read back from the disk). As shown in FIG. 2, a sampling timing error of 15 nanoseconds causes a sampled measurement error at 6X, which is significantly greater than at 1X.
A mark formation effectiveness measurement method described in commonly-assigned U.S. Pat. No. 5,495,466, to Dohmeier et al., the disclosure of which is herein incorporated by reference, effectively solves this timing error problem for a reflected write signal which has a well defined peak. Such a signal occurs, for example, with CD-R media. Rather than sampling the reflected write signal, this method works by dynamically adjusting one or more threshold levels relative to the reflected write signal. Each threshold level is adjusted until the reflected write signal exceeds that threshold for a predetermined time period. Referring to FIG. 3, a graph of a reflected write signal versus time is shown which illustrates the mark formation effectiveness measurement method disclosed in U.S. Pat. No. 5,495,466. In FIG. 3, the threshold level V.sub.1 is adjusted so that the reflected write signal exceeds it for a predetermined time period .DELTA.T.sub.1. Similarly, the threshold level V.sub.2 is adjusted so that the reflected write signal exceeds it for a predetermined time period .DELTA.T.sub.2. Mark formation effectiveness measurements can be made by processing the threshold levels V.sub.1 and V.sub.2, which is described in more detail in U.S. Pat. No. 5,495,466. This method accomplishes accurate mark formation effectiveness measurements without needing to provide the precise sample timing of the reflected write signal, as required by U.S. Pat. No. 5,216,660. However, one of the limitations of the method described in U.S. Pat. No. 5,495,466 is its speed of response. Since the threshold levels are dynamically adjusted, the threshold levels do not instantaneously follow changes in the reflected write signal. This time lag can be a problem, for example, when measuring rapid waveform fluctuations such as occur at the wobble frequency of CD-R.